1. Field of the Invention
The present invention relates to a two-pole overcurrent protection device for connection into a current-carrying conductor. The protection device comprises a normally conducting turn-off thyristor having a semiconductor body with, in successive order, a first emitter layer, a first base layer, a second base layer and a second emitter layer. An MOS transistor is integrated with the thyristor and is parallel-connected to the PN junction of the latter between the first emitter layer and the first base layer. When the voltage across the thyristor exceeds a predetermined value, the MOS transistor short-circuits the PN junction and causes the thyristor to cease conducting.
2. Description of the Prior Art
Overcurrent protection devices of the above-mentioned kind are previously known from Svedberg U.S. Pat. No. 4,331,884 (issued May 25, 1982) and U.S. patent application Ser. No. 497,326 (applied for on the May 23, 1983 by Per Svedberg and assigned to the assignee of this application). Such known overcurrent protection devices comprise a thyristor, whose cathode-emitter junction can be shorted out by an MOS transistor integrated with the thyristor. The control electrode of the thyristor is connected to the anode-emitter via a resistor formed in the thyristor.
During normal operation, the thyristor receives a gate trigger current via this resistor and is made conducting as soon as its off-state voltage reaches a low positive value. The anode voltage of the thyristor is supplied to the control electrode of the normally non-conducting MOS transistor. The on-state voltage drop of the thyristor increases with increasing current, and, at a certain value of the thyristor current, the voltage of the control electrode of the transistor reaches the threshold voltage of the transistor. The transistor then becomes conducting and short-circuits the cathode-emitter junction of the thyristor, whereby the thyristor is turned off interrupting the current flow through the thyristor.
The thyristor then remains non-conducting until the voltage applied across the thyristor is again reduced to a low value, which, for example in the case of AC operation, occurs in the next cycle with the subsequent zero passage of the voltage.
In the above-mentioned known type of overcurrent protection device, the control electrode of the thyristor, as previously mentioned, is supplied with a gate trigger current from the anode via a resistor integrated with the thyristor. To obtain reliable ignition of the thyristor at a low off-state voltage, it is necessary for the resistance of this resistor not to be too high.
A sufficiently low resistance value, however, causes a high leakage current to flow through the resistor when the thyristor has been turned off to avoid an overcurrent and, therefore, the thyristor voltage is high. This leakage current gives rise to high losses in the overcurrent protection device and to insufficient isolation between the driving voltage and the object being protected by the device.